1. Field of the Invention
The present invention relates to a transmitter, receiver, and radio communications system and method. More particularly, the present invention relates to a transmitter, receiver, and radio communications system and method with radio frequency control mechanisms.
2. Description of the Related Art
Recent years have seen a rapid increase in demands for mobile communication services including cellular phones systems. In our highly developed information age society, radio communication technologies have become more and more important. While such systems use radio waves with very high frequencies, most radio communication devices are designed basically to process signals with lower frequencies because they are easier to handle.
In a typical radio transmitter, a given information signal modulates an intermediate frequency (IF) signal, and this modulated IF signal is up-converted to a radio frequency (RF) signal through a mixing process with a high frequency signal supplied from a local oscillator. The resultant radio frequency signal is then radiated into the air from a transmitter antenna. At the receiving end, the incoming radio frequency signal is received with a receiver antenna. By mixing it with its own local oscillator signal, the receiver down-converts the signal to the intermediate frequency again. Note here that the same local oscillator frequency has to be used at both ends, so that the receiving party can reproduce the original information signal correctly.
FIG. 17 shows a conventional radio communications system. This radio communications system 300 includes a transmitter 600 and a receiver 700. The transmitter 600 has a mixer 601, a local oscillator 602, an amplifier 603, and an antenna 604. The receiver 700 has an antenna 704, an amplifier 703, a mixer 701, and a local oscillator 702.
Referring to the transmitter 600, the mixer 601 receives a signal IFin having an intermediate frequency of fIF, which is modulated with an information signal. It also receives a local oscillator signal Loa with a frequency of fL from the local oscillator 602. The multiplication of these two input signals yields a transmission signal with a radio frequency of fRF, where fRF=fIF+fL. This radio frequency signal is boosted by the amplifier 603 and radiated from the antenna 604 into the air.
Referring to the receiver 700, the antenna 704 catches the radio frequency signal sent from the transmitter 600, which is then supplied to the amplifier 703 for signal amplification. The mixer 701 combines the amplified RF signal with a local oscillator signal Lob with the same frequency fL as used in the transmitter 600, thereby yielding an output signal Ifout. This output signal Ifout has the difference frequency (fRF−fL) between the two signals being mixed, which should be the same intermediate frequency fIF as that in the transmitter 600 because fRF−fL=(fIF+fL)−fL=fIF.
In order to make the above system work properly, the intermediate frequency signal IFout reproduced by the receiver mixer 701 agrees with the original intermediate frequency signal IFin used in the transmitter 600. This means that the local oscillator in the receiver 700 must have the same frequency as that in the transmitter 600. In other words, the two local oscillator signal Loa and Lob generated by the two local oscillators 602 and 702 have to agree with each other in terms of the frequency. Normally, this requirement is fulfilled by employing a phase-locked loop (PLL) circuit that generates an accurate local oscillator signal with a high frequency stability and low phase noise.
Meanwhile, regarding the frequency usage for radio communications, the millimeter band (30-300 GHz) and frequencies slightly below that band (called “quasi millimeter band”) are of great interest as a resource to serve the increasing user demand in recent years. These frequency bands have been little explored and are now considered to be particularly suitable for vehicle-to-vehicle communications such as Intelligent Transport Systems (ITS), or for sophisticated applications of radio communication technologies such as wireless LANs, because they do require a broader bandwidth to achieve high-speed data exchange. It should also be mentioned that millimeter-wave radio communications products targeted to general consumers have to be small and inexpensive.
The above-described conventional system 300 can operate properly in frequency ranges up to about 3 GHz, since it is relatively easy to design a stable local oscillator for those frequency ranges. However, the desired frequency range (i.e., millimeter band) is ten times or one hundred times as high as the operating range of conventional circuits. Because of the limited stability and accuracy of local oscillators, it is hard to upgrade the conventional transmitters and receivers for the purpose of millimeter-wave radio communication.
More technically, think of a voltage controlled oscillator (VCO) operating at frequencies over 30 GHz. This VCO must have an integrated PLL circuit to stabilize its output phase and frequency to the extent that is required in the millimeter band applications. However, it is difficult to provide such PLL circuits because they require a good divider and phase detector devices that can operate at extremely high frequencies. While millimeter wave applications require small, inexpensive, and accurate VCO devices, as mentioned above, none of the currently available components satisfy those requirements.
Another possible method to obtain a high frequency signal is to use frequency multipliers. With such devices, a stable oscillator signal in the order of a few GHz can be multiplied up to a designed frequency. However, the phase noise would also be multiplied to an intolerable level, causing communication errors in data transport.